The largest-ever study of the long-term consequences of premature birth finds that children born early have higher death rates in childhood and are more likely to be childless as adults.

Experts called the research significant because it followed 1.2 million Norwegian births over decades. It also raises questions about future risks for even tinier babies saved today by modern medicine.

New drugs and therapies first used widely in the 1990s now save smaller and sicker babies. So the babies in the study may have been healthier, on average, than children born premature in recent years, experts said.

Most preemies grow up to have good health and normal reproduction, but the researchers found heightened risks compared with babies born at full term from 1967 to 1988. The findings add to known consequences of prematurity such as lung problems, disabilities, mental retardation and school delays.

U.S. rates of premature births climbed steadily during the past two decades, reaching an estimated 12.8 percent of births in 2006, government figures show. More than 540,000 babies were born premature that year. Fertility treatments that result in multiple births and older mothers contributed to the rise. (The rate of premature births in Norway that year was 7 percent.)

"In the United States, there is an epidemic of preterm birth, and prevention is absolutely critical," said Dr. Alan Fleischman of the March of Dimes. He was not involved in the study.

Fleischman said prevention efforts include hormone treatment for women with a history of giving premature birth, avoiding inducing labor unless medically necessary and reducing the number of embryos implanted at one time during fertility treatments.

As expected, babies born early were more likely to die during the first year of life compared with babies born at term.

Surprisingly, their increased risk of death persisted as they aged.

The children who were born five to nine weeks early (28 to 32 weeks into the pregnancy) showed a doubled risk of death from ages 1 to 5 compared with children born at normal term. (The overall risk of death was low: 33 of the 5,880 children in the premature group died.)

When the researchers looked at boys and girls separately, they found a stronger link in boys between premature birth and higher death rates in childhood.

The causes of childhood deaths are still being analyzed, but birth defects and childhood cancers played a role, Swamy said.

In adulthood, other differences showed up. Prematurity was linked to lower levels of education and more childlessness in both men and women in a subset of more than 580,000 births from 1967 through 1976.

Women who were preemies had a higher risk of giving birth to premature babies themselves. The risk of next-generation premature births increased with the severity of prematurity in the mothers.